Low Poly Water Effect
It looks to me like some kind of mix between glossy and transparent shaders.


This setup is rather arbitrary, it's up to you to come up with a combo you like.

The trick is that there seems to be multiple layers of mesh superimposed, allowing objects near the surface to be seen, while nearly completely obscuring the bottom.

One way to go about creating such layers is by first creating a "base mesh". I used a fluid simulation, but for more control you may want to use dyntopo.

Once you have a mesh more or less how you'd like it, duplicate it and add differently seeded displace modifiers, and decimate modifiers with different ratios to get a mix of polygon sizes.





Also note the way the water meshes in your example have been sculpted to give the impression of currents; long poly strips where water is flowing straight, and choppier, evenly distributed triangles for more turbulent regions.
I didn't do this, but you may want to.